Field of the Invention
The present invention relates to wastewater treatment and resource recovery processes and systems. In particular, this invention relates to the process and system for P and N removal from wastewater and recovery as usable materials by applying dolomite lime and slated dolomite lime.
Background of the Related Art
Municipal and industrial wastewater effluent, runoff and infiltration from agricultural and industrial sites, and other point and nonpoint sources increase the flux of P and N nutrients into terrestrial and aquatic ecosystems. The effluent of wastewater treatment plant (WWTP) has been recognized as one of major sources of P and N pollution. Human activities have accelerated the rate at which nutrients enter ecosystems. Elevated concentrations of nutrients in the environment has resulted in eutrophication problems worldwide, with more than about 50% of the lakes in North America, Europe and Asia being eutrophic. Eutrophication results in enhanced growth of choking aquatic vegetation or phytoplankton, which disrupts normal functioning of the ecosystem, pollutes drinking water sources, and interferers with drinking water treatment.
On the other hand, P and N element are essential nutrients for agricultural production. The existing phosphate rock resources can only maintain human demand for about 200 years. The sustainable development requires efficient P and N removal and recovery from wastewater as reusable material.
Currently, some WWTPs use P removal techniques, and a few of them have tested P recovery processes. The common methods for P removal are biological treatment (Liu et al., 1996; Lesjean et al., 2003) and chemical precipitation (Maurer and Boller, 1999; De-Bashan and Bashan, 2004). Biological P removal can be achieved via phosphate-accumulating organisms under anaerobic-aerobic conditions. However, it increases the cost and complexity for WWTP operation. Chemical precipitation uses salts of multivalent metal ions, such as calcium, aluminum and iron. Nonetheless, these additives generate hydroxide precipitates, which would increase the sludge volume and reduce sludge biodegradable property.
During wastewater treatment processes, most P and N are enriched and deposited in activated sludge (AS). Substantial amount of the deposited P and N would be released into solution when the AS is treated in the anaerobic digester. High concentration of phosphate and ammonium in anaerobic digestion effluent offers a good opportunity for the removal and recovery of these compounds. Currently, the technologies applied to remove and recover P from anaerobic digestion effluent are mainly based on the formation of phosphate precipitates (Momberg and Oellermann, 1992; Shu et al., 2006; Pastor et al., 2008; Song et al., 2007; He et al., 2007; Chen et al., 2009; Zhang et al., 2009; Ye et al., 2010; Song et al., 2011). Phosphate can be precipitated as struvite (MgNH4PO4.6(H2O)) via dosing magnesium salts, such as magnesium chloride. The addition of calcium compounds, such as lime, can form hydroxyapatite (Ca10(PO4)6(OH)2) precipitate with phosphate. Elevating the pH of anaerobic digestion effluent by addition of alkaline materials or by air sparging to remove carbon dioxide from the effluent can facilitate the formation of struvite as well.
U.S. Pat. No. 6,893,567 B1 discloses a biological nitrification process for conversion of ammonium to nitrate. Then magnesium chloride, calcium chloride and sodium hydroxide potassium are added into the pre-treated wastewater to promote the formation of phosphate precipitates.
U.S. Pat. No. 7,622,047 discloses a method for production of granular struvite using a fluidized bed reactor. Magnesium ion is added into the wastewater for the formation of struvite. The wastewater pH is increased to a range of 7.4-8.5 with addition of alkali and also by air stripping for removal of carbon dioxide.
U.S. Pat. No. 7,842,186 discloses a two-step process for formation of struvite precipitate in wastewater. Firstly, a wastewater containing phosphate and ammonium is treated with an anaerobic reactor. Then, the wastewater pH is increased from 8.4 to 9.6 by air stripping removal of carbon dioxide to form struvite precipitate.
In summary, the use of magnesium salts for precipitation of phosphates is expensive and application of lime is not efficient for phosphate removal and recovery. So far no technology has been developed for simultaneous removal and phosphate and ammonium with the same additive in a treatment process.